Zinc oxide varistor, a method of preparing the same, and a crystallized glass composition for coating

ABSTRACT

The present invention relates to a zinc oxide varistor as a characteristic element of an arrestor for protecting a transmission and distribution line and peripheral devices thereof from surge voltage created by lightning, and more particularly a highly reliable zinc oxide varistor excellent in the non-linearity with respect to voltage, the discharge withstand current rating properties, and the life characteristics under voltage, a method of preparing the same, and PbO type crystallized glass for coating oxide ceramics employed for a zinc oxide varistor, etc. A zinc oxide varistor of the present invention includes a sintered body (1) and a high resistive side layer (3) consisting of crystallized glass with high crystallinity containing the prescribed amount of SiO 2 , MoO 3 , WO 3 , TiO 2 , NiO, etc., formed on the sides of the sintered body (1) to enhance the strength and the insulating property thereof, thereby improving the non-linearity with respect to voltage, the discharge withstand current rating properties and the life characteristics under voltage. The crystallized glass composition for coating of the present invention includes PbO as a main component and additives such as ZnO, B 2  O 3  , SiO 2 , MoO 3 , WO 3 , TiO 2 , and NiO to enhance the crystallinity and the insulating property thereof.

TECHNICAL FIELD

The present invention particularly relates to a zinc oxide varistor usedin the field of an electric power system, a method of preparing thesame, and a crystallized glass composition used for coating an oxideceramic employed for a thermistor or a varistor.

BACKGROUND ART

A zinc oxide varistor comprising ZnO as a main component and severalkinds of metallic oxides including Bi₂ O₃, CoO, Sb₂ O₃, Cr₂ O₃, and MnO₂as other components has a high resistance to surge voltage and excellentnon-linearity with respect to voltage. Therefore, it has been generallyknown that the zinc oxide varistor is widely used as an element for agapless arrestor in place of conventional silicon carbide varistors inrecent years.

For example, Japanese Laid-open Patent Publication No. 62-101002, etc.,disclose conventional methods of preparing a zinc oxide varistor. Theaforesaid prior art reference discloses as follows: first, to ZnO as amain component are added metallic oxides such as Bi₂ O₃, Sb₂ O₃, Cr₂ O₃,CoO, and MnO₂ each in an amount of 0.01 to 6.0 mol % to prepare a mixedpowder. Then, the mixed powder thus obtained is blended and granulated.The resulting granules are molded by application of pressure in acylindrical form, after which the molded body is baked in an electricfurnace at 1200° C. for 6 hours. Next, to the sides of the sintered bodythus obtained are applied glass paste consisting of 80 percent by weightof PbO type frit glass containing 60 percent by weight of PbO, 20percent by weight of feldspar, and an organic binder by means of ascreen printing machine in a ratio of 5 to 500 mg/cm², followed bybaking treatment. Next, both end faces of the element thus obtained aresubjected to surface polishing and then an aluminum metallikon electrodeis formed thereon, thereby obtaining a zinc oxide varistor.

However, since a zinc oxide varistor prepared by the aforesaidconventional method employed screen printing, a high resistive sidelayer was formed with a uniform thickness. This led to an advantage inthat discharge withstand current rating properties did not largely varyamong varistors thus prepared, whereas since the high resistive sidelayer was made of composite glass consisting of PbO type frit glass andfeldspar, the varistor also had disadvantages as follows: the dischargewithstand current rating properties were poor, and the non-linearitywith respect to voltage lowered during baking treatment of glass,thereby degrading the life characteristics under voltage.

DISCLOSURE OF INVENTION

The present invention overcomes the above conventional deficiencies. Theobjectives of the present invention are to provide a zinc oxide varistorwith high reliability and a method of preparing the same. Anotherobjective of the present invention is to provide a crystallized glasscomposition suited for coating an oxide ceramic employed for a varistoror a thermistor.

In the present invention, for the purpose of achieving the aforesaidobjectives, to the sides of a sintered body comprising ZnO as a maincomponent is applied crystallized glass comprising PbO as a maincomponent such as PbO-ZnO-B₂ O₃ -SiO₂, MoO₃, WoO₃, NiO, Fe₂ O₃, or TiO₂type crystallized glass, followed by baking treatment, to form a highresistive side layer consisting of PbO type crystallized glass on thesintered body, thereby completing a zinc oxide varistor.

Furthermore, the present invention proposes a crystallized glasscomposition for coating an oxide ceramic comprising PbO as a maincomponent, and other components such as ZnO, B₂ O₃, SiO₂, MoO₃, WO₃,NiO, Fe₂ O₃, and TiO₂.

Since crystallized glass comprising PbO as a main component according tothe present invention has high strength of the coating film due to theaddition of SiO₂, MoO₃, WO₃, NiO, Fe₂ O₃, TiO₂, etc., and excellentadhesion to a sintered body, it has excellent discharge withstandcurrent rating properties and high insulating properties. This resultsin a minimum decline in non-linearity with respect to voltage duringbaking treatment to obtain a highly reliable zinc oxide varistor withexcellent life characteristics under voltage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross-sectional view of a zinc oxide varistor prepared byusing PbO type crystallized glass according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A zinc oxide varistor, a method of preparing the same, and acrystallized glass composition for coating according to the presentinvention will now be explained in detail by reference to the followingexamples.

EXAMPLE 1

First, to a ZnO powder were added 0.5 mol % of Bi₂ O₃, 0.5 mol % of Co₂O₃, 0.5 mol % of MnO₂, 1.0 mol % of Sb₂ O₃, 0.5 mol % of Cr₂ O₃, 0.5 mol% of NiO, and 0.5 mol % of SiO₂ based on the total amount of the mixedpowder. The resulting mixed powder was sufficiently blended and groundtogether with pure water, a binder, and a dispersing agent, for example,in a ball mill, after which the ground powder thus obtained was driedand granulated by means of a spray dryer to prepare a powder. Next, theresulting powder was subjected to compression molding to obtain a moldedpowder with a diameter of 40 mm and a thickness of 30 mm, followed bydegreasing treatment at 900° C. for 5 hours. Thereafter, the resultingmolded body was baked at 1150° C. for 5 hours to obtain a sintered body.

Alternatively, as for crystallized glass for coating, each predeterminedamount of PbO, ZnO, B₂ O₃, and SiO₂ was weighed, and then mixed andground, for example, in a ball mill, after which the ground powder wasmelted at a temperature of 1100° C. and rapidly cooled in a platinumcrucible to be vitrified. The resulting glass was subjected to coarsegrinding, followed by fine grinding in a ball mill to obtain frit glass.On the other hand, as a control sample, composite glass consisting of80.0 percent by weight of frit glass consisting of 70.0 percent byweight of PbO, 25.0 percent by weight of ZnO, and 5.0 percent by weightof B₂ O₃, and 20.0 percent by weight of feldspar (feldspar is a solidsolution comprising KAlSi₃ O₈, NaAlSi₃ O₈, and CaAl₂ Si₂ O₈) wasprepared in the same process as described before. The composition, theglass transition point Tg, the coefficient of linear expansion α, andthe crystallinity of the frit glass prepared in the aforesaid manner areshown in Table 1 below.

The glass transition point Tg and the coefficient of linear expansion αshown in Table 1 were measured by means of a thermal analysis apparatus.As for the crystallinity, the conditions of glass surface were observedby means of a metallurgical microscope or an electron microscope, afterwhich a sample with high crystallinity was denoted by a mark "o", asample with low crystallinity a mark "Δ", and a sample with no crystal amark "x".

                  TABLE 1                                                         ______________________________________                                               Composition                                                            Name of                                                                              (Percent by weight)                                                                           Tg     α Crystal-                                glass  PbO    ZnO    B.sub.2 O.sub.3                                                                    SiO.sub.2                                                                          (°C.)                                                                       (10.sup.-7 /°C.)                                                               linity                            ______________________________________                                        G101*  40     25     10   25   470  61      ◯                     G102   50     25     10   15   456  68      ◯                     G103   60     15     10   15   432  79      ◯                     G104   75     15     5    10   385  85      ◯                     G105*  80      5     5    10   380  93      X                                 G106*  60     10     5    25   363  70      ◯                     G107   60     15     5    20   375  66      ◯                     G108   60     29     5    6    404  72      ◯                     G109*  60     35     15   0    409  69      ◯                     G110*  65     25     2.5  7.5  351  73      ◯                     G111   62.5   25     5    7.5  388  75      ◯                     G112   57.5   25     10   7.5  380  70      ◯                     G113*  52.5   25     15   7.5  427  66      X                                 G114*  66     20     10   4    350  79      ◯                     G115   64     20     10   6    374  75      ◯                     G116   60     20     10   10   396  70      ◯                     G117   55     20     10   15   402  66      ◯                     G118*  50     20     10   20   448  59      X                                 ______________________________________                                         A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

As shown in Table 1, the addition of a large amount of PbO raises thecoefficient of linear expansion α, while the addition of a large amountof ZnO lowers the glass transition point Tg, which facilitatescrystallization of the glass composition. Conversely, the addition of alarge amount of B₂ O₃ raises the glass transition point, and theaddition of more than 15.0 percent by weight of B₂ O₃ causes difficultyin crystallization of the glass composition. Further, with an increasein the amount of SiO₂ added, the glass transition point tends toincrease, while the coefficient of linear expansion tends to decrease.

Next, 85 percent by weight of the frit glass of the aforementionedsample and 15 percent by weight of a mixture of ethyl cellulose andbutyl carbitol acetate as an organic binder were sufficiently mixed, forexample, by a triple roll mill, to obtain glass paste for coating. Theglass paste for coating thus obtained was printed on the sides of theaforesaid sintered body by means of, for example, a screen printingmachine for curved surface with a screen of 125 to 250 mesh. In thisprocess, the amount of the glass paste for coating to be applied wasdetermined by measurement of a difference in weight between the sinteredbodies prior and posterior to a process for coating with paste anddrying for 30 minutes at 150° C. The amount of the glass paste forcoating to be applied was also adjusted by adding an organic binder andn-butyl acetate thereto. Thereafter, the glass paste for coating wassubjected to baking treatment at temperatures in the range of 350° to700° C. to form a high resistive side layer on the sides of the sinteredbody. Next, the both end faces of the sintered body were subjected tosurface polishing, and then an aluminum metallikon electrode was formedthereon, thereby obtaining a zinc oxide varistor.

FIG. 1 shows a cross-sectional view of a zinc oxide varistor obtained inthe aforesaid manner according to the present invention. In FIG. 1, thereference numeral 1 denotes a sintered body comprising zinc oxide as amain component, 2 an electrode formed on both end faces of the sinteredbody 1, and 3 a high resistive side layer obtained by a process forbaking crystallized glass on the sides of the sintered body 1.

Next, the appearance, V_(1mA) /V.sub.μA, the discharge withstand currentrating properties, and the life characteristics under voltage of a zincoxide varistor prepared by using the glass for coating shown in Table 1above are shown in Table 2 below. The viscosity of the glass paste forcoating was controlled so that the paste could be applied in a ratio of50 mg/cm². The baking treatment was conducted at a temperature of 550°C. for 1 hour. Each lot has 5 samples. V_(1mA) /V₁₀μA was measured byusing a DC constant-current source. The discharge withstand currentrating properties were examined by applying an impulse current of 4/10μS to each sample at five-minute intervals in the same direction twiceand stepping up the current from 40 kA. Then, whether any unusualappearance was observed or not was examined visually, or, if necessary,by means of a metallurgical microscope. In the Table, the mark "o"denotes that no unusual appearance was observed in a sample after theprescribed electric current was applied to the sample twice. The mark"Δ" and "x" denote that unusual appearance was observed in 1 to 2samples, and 3 to 5 samples, respectively. Further, with the lifecharacteristics under voltage, the time required for leakage current toreach 5 mA, i.e., a peak value was measured at ambient temperature of130° C. and a rate of applying voltage of 95% (AC, peak value). V_(1mA)/V₁₀μA and the life characteristics under voltage are represented by anaverage of those of 5 samples.

The number of samples, the method of measuring V_(1mA) /V₁₀μA, themethod of testing the discharge withstand current rating, and the methodof evaluating the life characteristics under voltage described abovewill be adopted unchanged in each following examples unless otherwisestated.

                                      TABLE 2                                     __________________________________________________________________________                         Life under                                                                          Discharge withstand current                        Name of              voltage                                                                             rating properties                                  glass  Appearance                                                                           V.sub.1mA /V.sub.10μA                                                             (Time)                                                                              40 kA                                                                             50 kA                                                                             60 kA                                                                             70 kA                                                                             80 kA                              __________________________________________________________________________    G101*  Partially                                                                            1.15   185   X   --  --  --  --                                        peel off                                                               G102   Good   1.21   206   ◯                                                                     ◯                                                                     ◯                                                                     X   --                                 G103   Good   1.23   370   ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                           X                                  G104   Good   1.34   320   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G105*  Crack  1.19    96   X   --  --  --  --                                 G106   Porous 1.16   340   Δ                                                                           X   --  --  --                                 G107   Good   1.18   314   ◯                                                                     ◯                                                                     ◯                                                                     X   --                                 G108   Good   1.25   291   ◯                                                                     ◯                                                                     X   --  --                                 G109*  Good   1.38   158   ◯                                                                     X   --  --  --                                 G110*  Good   1.20   369   ◯                                                                     ◯                                                                     X   --  --                                 G111   Good   1.21   351   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G112   Good   1.19   332   ◯                                                                     ◯                                                                     ◯                                                                     X   --                                 G113*  Porous 1.18   345   Δ                                                                           X   --  --  --                                 G114*  Good   1.34   171   ◯                                                                     ◯                                                                     X   --  --                                 G115   Good   1.25   243   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     X                                  G116   Good   1.21   297   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     Δ                            G117   Good   1.19   495   ◯                                                                     ◯                                                                     ◯                                                                     X   --                                 G118*  Peel off                                                                             1.17   331   X   --  --  --  --                                 Conventional                                                                         Good   1.26   153   ◯                                                                     Δ                                                                           X   --  --                                 example                                                                       __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

The data shown in Tables 1 and 2 indicated that when the coefficient oflinear expansion of glass for coating was smaller than 65×10⁻⁷ /° C.(G101, G118 glass), the glass tended to peel off, and when exceeding90×10⁻⁷ /° C., the glass tended to crack. It is also confirmed that thesamples of glass which cracked or peeled off have poor dischargewithstand current rating properties due to the inferior insulatingproperties of the high resistive side layer. However, even if thecoefficient of linear expansion of glass for coating is within the rangeof 65×10⁻⁷ to 90×10⁻⁷ /° C., glass with poor crystallinity (G105, G113glass) tends to crack and also has poor discharge withstand currentrating properties. This may be attributed to the fact that the coatingfilm of crystallized glass has lower strength than that of noncrystalglass. The addition of ZnO as a component of crystallized glass isuseful for the improvement of the physical properties, especially, adecrease in the glass transition point of glass without largelyaffecting the various electric characteristics and the reliability of azinc oxide varistor. It is also confirmed that when conventionalcomposite glass consisting of PbO-ZnO-B₂ O₃ glass and feldspar, i.e., acontrol sample, is used, the life characteristics under voltage is at apractical level, while the discharge withstand current rating propertiesare poor.

The amount of SiO₂ added will now be considered. First, any compositionwith less than 6.0 percent by weight of SiO₂ added has inferior lifecharacteristics under voltage. This may be attributed to the fact thatthe addition of less than 6.0 percent by weight of SiO₂ lowers theinsulation resistance of the coating film. On the other hand, theaddition of more than 15.0 percent by weight of SiO₂ lowers thedischarge withstand current rating properties. This may be attributed tothe fact that glass tends to become porous due to its poor fluidityduring the baking process. Consequently, a crystallized glasscomposition comprising PbO as a main component for the high resistiveside layer of a zinc oxide varistor is required to comprise SiO₂ atleast in an amount of 6.0 to 15.0 percent by weight.

The above results confirmed that the most preferable crystallized glasscomposition for coating comprised 50.0 to 75.0 percent by weight of PbO,10.0 to 30.0 percent by weight of ZnO, 5.0 to 10.0 percent by weight ofB₂ O₃, and 6.0 to 15.0 percent by weight of SiO₂. A crystallized glasscomposition for the high resistive side layer of a zinc oxide varistoris also required to have coefficients of linear expansion in the rangeof 65×10⁻⁷ to 90×10⁻⁷ /° C.

Next, by the use of G111 glass shown as a sample of the presentinvention in Table 1, the amount of glass paste to be applied wasexamined. The results are shown in Table 3 below. Glass paste wasapplied in a ratio of 1.0 to 300.0 mg/cm², which was controlled by theviscosity and the number of application of the paste. As shown in Table3, when glass paste is applied in a ratio of less than 10.0 mg/cm², theresulting coating film has low strength, while with a ratio of more than150.0 mg/cm², glass tends to have pin-holes. Both cases result in poordischarge withstand current rating properties. These results confirmedthat glass paste was applied most preferably in a ratio of 10.0 to 150.0mg/cm².

                                      TABLE 3                                     __________________________________________________________________________        Amount of           Life under                                                                          Discharge withstand current                     Sample                                                                            application         voltage                                                                             rating properties                               No. (mg/cm.sup.2)                                                                       Appearance                                                                           V.sub.1mA /V.sub.10μA                                                             (Time)                                                                              40 kA                                                                             50 kA                                                                             60 kA                                                                             70 kA                                                                             80 kA                           __________________________________________________________________________    101*                                                                               1    Good   1.14   367   X   --  --  --  --                              102*                                                                               3    Good   1.15   354   Δ                                                                           X   --  --  --                              103*                                                                               5    Good   1.20   360   Δ                                                                           X   --  --  --                              104  10   Good   1.23   394   ◯                                                                     ◯                                                                     Δ                                                                           X   --                              105  50   Good   1.21   351   ◯                                                                     ◯                                                                     Δ                                                                           X   --                              106 150   Good   1.28   308   ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                           X                               107*                                                                              200   Partially                                                                            1.33   269   ◯                                                                     X   --  --  --                                        flow                                                                108*                                                                              300   Flow   1.30   245   X   --  --  --  --                              __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

Next, by the use of Glll glass shown as a sample of the presentinvention in Table 1, the conditions under which glass paste wassubjected to baking treatment were examined. The results are shown inTable 4 below. The viscosity of glass paste was controlled so that theglass paste may be applied in a ratio of 50.0 mg/cm². Glass paste wassubjected to baking treatment at temperatures in the range of 350° to700° C. for 1 hour in air. Apparent from Table 4, when baking treatmentwas conducted at a temperature of less than 450° C., glass was notsufficiently melted, resulting in poor discharge withstand currentrating properties. On the other hand, when baking treatment wasconducted at a temperature of more than 650° C., the voltage ratiomarkedly lowered, resulting in poor life characteristics under voltage.These results indicated that glass paste was subjected to bakingtreatment most preferably at temperatures in the range of 450° to 650°C. It was also confirmed that the baking treatment conducted for 10minutes or more had no serious effect on various characteristics.

                                      TABLE 4                                     __________________________________________________________________________        Temperature         Life under                                                                          Discharge withstand current                     Sample                                                                            of baking           voltage                                                                             rating properties                               No. (°C.)                                                                         Appearance                                                                          V.sub.1mA /V.sub.10μA                                                             (Time)                                                                              40 kA                                                                             50 kA                                                                             60 kA                                                                             70 kA                                                                             80 kA                           __________________________________________________________________________    111*                                                                              350    Not   1.08    51   X   --  --  --  --                                         sintered                                                           112*                                                                              400    Porous                                                                              1.12    77   Δ                                                                           X   --  --  --                              113 450    Good  1.24   224   ◯                                                                     ◯                                                                     Δ                                                                           X   --                              114 500    Good  1.21   365   ◯                                                                     ◯                                                                     Δ                                                                           X   --                              115 600    Good  1.33   408   ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                           X                               116 650    Good  1.40   215   ◯                                                                     ◯                                                                     ◯                                                                     X   --                              117*                                                                              700    Partially                                                                           1.79    19   ◯                                                                     X   --  --  --                                         flow                                                               __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

EXAMPLE 2

Crystallized glass comprising PbO as a main component which containsMoO₃, and a zinc oxide varistor using the same as a materialconstituting a high resistive side layer will now be explained.

First, each predetermined amount of PbO, ZnO, B₂ O₃, SiO₂, and MoO₃ wasweighed, and then crystallized glass for coating was prepared accordingto the same process as that used in Example 1 described before. Theresults are shown in Table 5 below.

                                      TABLE 5                                     __________________________________________________________________________    Name of                                                                            Composition (percent by weight)                                                                Tg α                                                                             Crystal-                                       glass                                                                              PbO                                                                              ZnO B.sub.2 O.sub.3                                                                  SiO.sub.2                                                                        MoO.sub.3                                                                         (°C.)                                                                     (10.sup.-7 /°C.)                                                             linity                                         __________________________________________________________________________    G201*                                                                              40 25   5 10 20  349                                                                              61    ◯                                  G202 50 25   5 10 10  355                                                                              75    ◯                                  G203 75 10   5 5  5   336                                                                              88    ◯                                  G204*                                                                              85 10   5 0  0   315                                                                              96    X                                              G205*                                                                              55 40   5 0  0   350                                                                              60    ◯                                  G206 55 30  10 0  5   355                                                                              67    ◯                                  G207 70  5  15 5  5   366                                                                              75    Δ                                        G208*                                                                              70  0  20 5  5   375                                                                              87    X                                              G209 67.5                                                                             20  10 0  2.5 378                                                                              79    ◯                                  G210 67.4                                                                             20  10 0.1                                                                              2.5 382                                                                              80    ◯                                  G211 62.5                                                                             20  10 5  2.5 388                                                                              75    ◯                                  G212 57.5                                                                             20  10 10 2.5 400                                                                              73    ◯                                  G213*                                                                              47.5                                                                             20  10 20 2.5 405                                                                              68    ◯                                  G214*                                                                              59.99                                                                            20  10 10 0.01                                                                              395                                                                              70    ◯                                  G215 59.9                                                                             20  10 10 0.1 398                                                                              69    ◯                                  G216 55 20  10 10 5   404                                                                              72    ◯                                  G217 50 20  10 10 10  405                                                                              68    ◯                                  G218*                                                                              45 20  10 10 15  410                                                                              62    ◯                                  __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

As shown in Table 5, the addition of a large amount of PbO raises thecoefficient of linear expansion (α), while the addition of a largeamount of ZnO lowers the glass transition point (Tg), which facilitatescrystallization of the glass composition. Conversely, the addition of alarge amount of B₂ O₃ raises the glass transition point, and theaddition of more than 15.0 percent by weight of B₂ O₃ causes difficultyin crystallization of the glass composition. Further, with an increasein the amount of SiO₂ added, the glass transition point tends toincrease, while the coefficient of linear expansion tends to decrease.With an increase in the amount of MoO₃ added, the crystallization ofglass proceeded. The glass composition comprising a small amount of PbOand B₂ O₃ tended to become porous.

Next, the aforesaid frit glass was made into paste, after which theresulting glass paste was applied to the sides of the sintered body ofExample 1, followed by baking treatment to prepare a sample of a zincoxide varistor in the same process as that used in the above example.Thereafter, the resulting samples were evaluated for theircharacteristics.

The results are shown in Table 6 below.

                                      TABLE 6                                     __________________________________________________________________________                               Discharge withstand current                        Name of              Life under                                                                          rating properties                                  glass  Appearance                                                                           V.sub.1mA /V.sub.10μA                                                             voltage                                                                             40 kA                                                                             50 kA                                                                             60 kA                                                                             70kA                                                                              80kA                               __________________________________________________________________________    G201*  Peel off                                                                             1.16   352   X   --  --  --  --                                 G202   Good   1.17   450   ◯                                                                     ◯                                                                     ◯                                                                     X   --                                 G203   Good   1.23   381   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G204*  Crack  1.55    15   X   --  --  --  --                                 G205*  Partially                                                                            1.31   181   Δ                                                                           X   --  --  --                                        peel off                                                               G206   Good   1.20   319   ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                           X                                  G207   Good   1.19   485   ◯                                                                     ◯                                                                     X   --  --                                 G208*  Partially                                                                            1.31   238   X   --  --  --  --                                        crack                                                                  G209   Good   1.29   256   ◯                                                                     X   --  --  --                                 G210   Good   1.28   363   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G211   Good   1.23   472   ◯                                                                     ◯                                                                     ◯                                                                     X   --                                 G212   Good   1.20   550   ◯                                                                     ◯                                                                     X   --  --                                 G213*  Porous 1.18   316   X   --  --  --  --                                 G214*  Good   1.34   230   Δ                                                                           X   --  --  --                                 G215   Good   1.17   434   ◯                                                                     ◯                                                                     X   --  --                                 G216   Good   1.15   890   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     X                                  G217   Good   1.13   950   ◯                                                                     ◯                                                                     ◯                                                                     X   --                                 G218*  Porous 1.21   241   X   --  --  --  --                                 Convention                                                                           Good   1.26   153   ◯                                                                     Δ                                                                           X   --  --                                 example                                                                       __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

The data shown in Tables 5 and 6 indicated that when the coefficient oflinear expansion of glass for coating was smaller than 65×10⁻⁷ /° C.(G201, G205, G218 glass), the glass tended to peel off, and whenexceeding 90×10⁻⁷ /° C. (G204 glass), the glass tended to crack. It issupposed that the samples of glass which cracked or peeled off have poordischarge withstand current rating properties due to the inferiorinsulating properties of the high resistive side layer. However, even ifthe coefficient of linear expansion of glass for coating is within therange of 65×10⁻⁷ to 90×10⁻⁷ /° C., glass with poor crystallinity (G208glass) tends to crack and also has poor discharge withstand currentrating properties. This may be attributed to the fact that the coatingfilm of crystallized glass has higher strength than that of non-crystalglass.

The amount of MoO₃ added will now be considered. First, any compositionwith 0.1 percent by weight or more of MoO₃ added has improvednon-linearity with respect to voltage, accompanied by the improved lifecharacteristics under voltage. This may be attributed to the fact thatthe addition of 0.1 percent by weight or more of MoO₃ raises theinsulation resistance of the coating film. On the other hand, theaddition of more than 10.0 percent by weight of MoO₃ lowers thedischarge withstand current rating properties. This may be attributed tothe fact that glass tends to become porous due to its poor fluidityduring baking process. Consequently, a PbO-ZnO-B₂ O₃ -SiO₂ -MoO₃ typecrystallized glass composition for the high resistive side layer of azinc oxide varistor is required to comprise MoO₃ at least in an amountof 0.1 to 10.0 percent by weight.

The above results confirmed that the most preferable crystallized glasscomposition for coating comprised 50.0 to 75.0 percent by weight of PbO,10.0 to 30.0 percent by weight of ZnO, 5.0 to 10.0 percent by weight ofB₂ O₃, 0 to 15.0 percent by weight of SiO₂, and 0.1 to 10.0 percent byweight of MoO₃. The crystallized glass composition for the highresistive side layer of a zinc oxide varistor is also required to havecoefficients of linear expansion in the range of 65×10⁻⁷ to 90×10⁻⁷ /°C.

Next, by the use of G206 glass shown as a sample of the presentinvention in Table 5, the amount of glass paste to be applied wasexamined. The results are shown in Table 7 below. Glass paste wasapplied in a ratio of 1.0 to 300.0 mg/cm², which was controlled by theviscosity and the number of application of the paste. As shown in Table7, when glass paste is applied in a ratio of less than 10.0 mg/cm², theresulting coating film has low strength, while with a ratio of more than150.0 mg/cm², glass tends to flow or have pinholes. Both cases result inpoor discharge withstand current rating properties. These resultsindicated that glass paste was applied most preferably in a ratio of10.0 to 150.0 mg/cm².

                                      TABLE 7                                     __________________________________________________________________________        Amount of           Life under                                                                           Discharge withstand current                    Sample                                                                            application         voltage                                                                              rating properties                              No. (mg/cm.sup.2)                                                                       Appearance                                                                           V.sub.1mA /V.sub.10μA                                                             (Time) 40 kA                                                                             50 kA                                                                             60 kA                                                                             70 kA                                                                            80 kA                           __________________________________________________________________________    201*                                                                               1    Good   1.10   318    X   --  --  -- --                              202*                                                                               5    Good   1.13   364    Δ                                                                           X   --  -- --                              203  10   Good   1.14   913    ◯                                                                     ◯                                                                     ◯                                                                     X  --                              204  50   Good   1.15   890    ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                    X                               205 150   Good   1.20   592    ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                          X                               206*                                                                              200   Partially                                                                            1.29   387    ◯                                                                     X   --  -- --                                        flow                                                                207*                                                                              300   Flow   1.30   311    X   --  --  -- --                              __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

Next, by the use of G206 glass shown as a sample of the presentinvention in Table 5, the conditions under which glass paste wassubjected to baking treatment were examined. The results are shown inTable 8 below. The viscosity of glass paste was controlled so that theglass paste may be applied in a ratio of 50.0 mg/cm². Glass paste wassubjected to baking treatment at temperatures in the range of 350° to700° C. for 1 hour in air. As a result, when baking treatment wasconducted at a temperature of less than 450° C., glass paste was notsufficiently melted, resulting in poor discharge withstand currentrating properties. On the other hand, when baking treatment wasconducted at a temperature of more than 650° C., the voltage ratiomarkedly lowered, resulting in poor life characteristics under voltage.These results indicated that glass paste was subjected to bakingtreatment most preferably at temperatures in the range of 450° to 650°C.

                                      TABLE 8                                     __________________________________________________________________________        Temperature         Life under                                                                           Discharge withstand current                    Sample                                                                            of baking           voltage                                                                              rating properties                              No. (°C.)                                                                         Appearance                                                                          V.sub.1mA /V.sub.10μA                                                             (Time) 40 kA                                                                             50 kA                                                                             60 kA                                                                             70 kA                                                                            80 kA                           __________________________________________________________________________    211*                                                                              350    Not   1.12    48    X   --  --  -- --                                         Sintered                                                           212*                                                                              400    Porous                                                                              1.13    52    X   --  --  -- --                              213 450    Good  1.15   431    ◯                                                                     ◯                                                                     X   -- --                              214 500    Good  1.15   980    ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                          X                               215 600    Good  1.22   850    ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                          X                               216 650    Good  1.32   452    ◯                                                                     ◯                                                                     X   -- --                              217*                                                                              700    Flow  1.76    5     X   --  --  -- --                              __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

EXAMPLE 3

Crystallized glass comprising PbO as a main component which containsWO₃, and a zinc oxide varistor using the same as a material constitutinga high resistive side layer will now be explained.

First, each predetermined amount of PbO, ZnO, B₂ O₃, SiO₂, and MoO₃ wasweighed, and then crystallized glass for coating was prepared accordingto the same process as that used in Example 1 described before. Thecrystallized glass thus obtained was evaluated for the glass transitionpoint (Tg), the coefficient of linear expansion (α), and thecrystallinity. The results are shown in Table 9 below.

                                      TABLE 5                                     __________________________________________________________________________    Name of                                                                            Composition (percent by weight)                                                                Tg α                                                                             Crystal-                                       glass                                                                              PbO                                                                              ZnO B.sub.2 O.sub.3                                                                  SiO.sub.2                                                                        WO.sub.3                                                                          (°C.)                                                                     (10.sup.-7 /°C.)                                                             linity                                         __________________________________________________________________________    G301*                                                                              40 25   5 10 20  355                                                                              60    ◯                                  G302 50 25   5 10 10  361                                                                              73    ◯                                  G303 75 10   5 5  5   340                                                                              89    ◯                                  G304*                                                                              85 10   5 0  0   315                                                                              96    X                                              G305*                                                                              50 40   5 5  0   342                                                                              62    ◯                                  G306 50 30  10 5  5   351                                                                              66    ◯                                  G307 65  5  15 5  5   372                                                                              73    X                                              G308*                                                                              70  0  20 5  5   384                                                                              88    X                                              G309*                                                                              67.4                                                                             20  10 0.1                                                                              2.5 380                                                                              81    ◯                                  G310 67.0                                                                             20  10 0.5                                                                              2.5 384                                                                              80    ◯                                  G311 62.5                                                                             20  10 5  2.5 392                                                                              76    ◯                                  G312 57.5                                                                             20  10 10 2.5 401                                                                              72    ◯                                  G313*                                                                              47.5                                                                             20  10 20 2.5 406                                                                              67    ◯                                  G314*                                                                              59.9                                                                             20  10 10 0.1 396                                                                              71    ◯                                  G315 59.5                                                                             20  10 10 0.5 399                                                                              72    ◯                                  G316 55 20  10 10 5   404                                                                              70    ◯                                  G317 50 20  10 10 10  405                                                                              68    ◯                                  G318*                                                                              45 20  10 10 15  412                                                                              66    ◯                                  __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

As shown in Table 9, the addition of a large amount of PbO raises thecoefficient of linear expansion, while the addition of a large amount ofZnO lowers the glass transition point (Tg), which facilitatescrystallization of the glass composition. Conversely, the addition of alarge amount of B₂ O₃ raises the glass transition point, and theaddition of more than 15.0 percent by weight of B₂ O₃ causes difficultyin crystallization of the glass composition. Further, with an increasein the amount of SiO₂ added, the glass transition point tends toincrease, while the coefficient of linear expansion tends to decrease.With an increase in the amount of WO₃ added, the crystallization ofglass proceeded.

Next, the aforesaid frit glass was made into paste, after which theresulting glass paste was applied to the sides of the sintered body ofExample 1, followed by baking treatment to prepare a sample of a zincoxide varistor in the same process as that used in Example 1 above.Thereafter, the resulting samples were evaluated for theircharacteristics.

The results are shown in Table 10 below.

                                      TABLE 10                                    __________________________________________________________________________                               Discharge withstand current                        Name of              Life under                                                                          rating properties                                  glass  Appearance                                                                           V.sub.1mA /V.sub.10μA                                                             voltage                                                                             40 kA                                                                             50 kA                                                                             60 kA                                                                             70kA                                                                              80kA                               __________________________________________________________________________    G301*  peel off                                                                             1.19   346   X   --  --  --  --                                 G302   Good   1.20   400   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G303   Good   1.30   292   ◯                                                                     ◯                                                                     ◯                                                                     X   --                                 G304*  Crack  1.55    15   X   --  --  --  --                                 G305*  Partially                                                                            1.36   142   X   --  --  --  --                                        Peel off                                                               G306   Good   1.24   280   ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                           X                                  G307   Good   1.21   397   ◯                                                                     Δ                                                                           X   --  --                                 G308*  Partially                                                                            1.34   221   X   --  --  --  --                                        crack                                                                  G309*  Good   1.31   260   ◯                                                                     X   --  --  --                                 G310   Good   1.29   334   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G311   Good   1.25   415   ◯                                                                     ◯                                                                     ◯                                                                     X   --                                 G312   Good   1.22   490   ◯                                                                     ◯                                                                     X   --  --                                 G313*  Porous 1.18   345   X   --  --  --  --                                 G314*  Good   1.35   247   ◯                                                                     X   --  --  --                                 G315   Good   1.29   330   ◯                                                                     ◯                                                                     X   --  --                                 G316   Good   1.18   451   ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                           X                                  G317   Good   1.15   600   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G318*  Porous 1.20   298   X   --  --  --  --                                 Conventional                                                                         Good   1.26   153   ◯                                                                     Δ                                                                           X   --  --                                 example                                                                       __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

The data shown in Tables 9 and 10 indicated that when the coefficient oflinear expansion of glass for coating was smaller than 65×10⁻⁷ /° C.(G301, G305 glass), the glass tended to peel off, and when exceeding90×10⁻⁷ /° C., the glass tended to crack. It is supposed that thesamples of glass which cracked or peeled off have poor dischargewithstand current rating properties due to the inferior insulatingproperties of the high resistive side layer. However, even if thecoefficient of linear expansion of glass for coating is within the rangeof 65×10⁻⁷ to 90×10⁻⁷ /° C., glass with poor crystallinity (G304, G308glass) tends to crack and also has poor discharge withstand currentrating properties. This may be attributed to the fact that the coatingfilm of crystallized glass has lower strength than that of noncrystalglass.

The amount of WO₃ added will now be considered. First, any compositionwith 0.5 percent by weight or more of WO₃ added has the improvednon-linearity with respect to voltage, accompanied by the improved lifecharacteristics under voltage. This may be attributed to the fact thatthe addition of 0.5 percent by weight or more of WO₃ raises theinsulation resistance of the coating film. On the other hand, theaddition of more than 10.0 percent by weight of WO₃ (G1 glass) lowersthe discharge withstand current rating properties. This may beattributed to the fact that glass tends to become porous due to its poorfluidity during baking process. Consequently, a crystallized glasscomposition comprising PbO as a main component for the high resistiveside layer of a zinc oxide varistor is required to comprise WO₃ at leastin an amount of 0.5 to 10.0 percent by weight.

The above results confirmed that the most preferable crystallized glasscomposition comprised 50.0 to 75.0 percent by weight of PbO, 10.0 to30.0 percent by weight of ZnO, 5.0 to 15.0 percent by weight of B₂ O₃,0.5 to 15.0 percent by weight of SiO₂, and 0.5 to 10.0 percent by weightof WO₃. A crystallized glass composition for the high resistive sidelayer of a zinc oxide varistor is also required to have coefficients oflinear expansion in the range of 65×10⁻⁷ /° C. to 90×10⁻⁷ /° C.

Next, by the use of G316 glass shown as a sample of the presentinvention in Table 9, the amount of glass paste to be applied wasexamined. The results are shown in Table 11 below. Glass paste wasapplied in a ratio of 1.0 to 300.0 mg/cm², which was controlled by theviscosity and the number of application of the paste. As shown in Table11, when glass paste is applied in a ratio of less than 10.0 mg/cm², theresulting coating film has low strength, while with a ratio of more than150.0 mg/cm², glass tends to have pinholes. Both cases result in poordischarge withstand current rating properties. These results indicatedthat glass paste was applied most preferably in a ratio of 10.0 to 150.0mg/cm².

                                      TABLE 11                                    __________________________________________________________________________        Amount of           Life under                                                                          Discharge withstand current                     Sample                                                                            application         voltage                                                                             rating properties                               No. (mg/cm.sup.2)                                                                       Appearance                                                                           V.sub.1mA /V.sub.10μA                                                             (Time)                                                                              40 kA                                                                             50 kA                                                                             60 kA                                                                             70 kA                                                                             80 kA                           __________________________________________________________________________    301*                                                                               1    Good   1.11   309   X   --  --  --  --                              302*                                                                               5    Good   1.13   362   Δ                                                                           X   --  --  --                              303  10   Good   1.14   578   ◯                                                                     ◯                                                                     Δ                                                                           X   --                              304  50   Good   1.18   451   ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                           X                               305 150   Good   1.21   490   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     X                               306*                                                                              200   Partially                                                                            1.28   300   ◯                                                                     X   --  --  --                                        flow                                                                307*                                                                              300   Flow   1.31   241   Δ                                                                           X   --  --  --                              __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

Next, by the use of G316 glass shown as a sample of the presentinvention in Table 9, the conditions under which glass paste wassubjected to baking treatment were examined. The results are shown inTable 12 below. The viscosity and the number of application of glasspaste were controlled so that the glass paste may be applied in a ratioof 50.0 mg/cm². Glass paste was subjected to baking treatment attemperatures in the range of 350° to 700° C. for 1 hour in air. Apparentfrom Table 12, when baking treatment was conducted at a temperature ofless than 450° C., glass paste was not sufficiently melted, resulting inpoor discharge withstand current rating properties. On the other hand,when baking treatment was conducted at a temperature of more than 600°C., the voltage ratio markedly lowered, resulting in poor lifecharacteristics under voltage. These results indicated that glass pastewas subjected to baking treatment most preferably at temperatures in therange of 450° to 600 ° C.

                                      TABLE 12                                    __________________________________________________________________________        Temperature          Life under                                                                          Discharge withstand current                    Sample                                                                            of baking            voltage                                                                             rating properties                              No. (°C.)                                                                         Appearance                                                                           V.sub.1mA /V.sub.10μA                                                             (Time)                                                                              40 kA                                                                             50 kA                                                                             60 kA                                                                             70 kA                                                                             80 kA                          __________________________________________________________________________    311*                                                                              350    Not    1.10    45   X   --  --  --  --                                        sintered                                                           312*                                                                              400    Porous 1.12    42   X   --  --  --  --                             313 450    Good   1.15   230   ◯                                                                     ◯                                                                     X   --  --                             314 500    Good   1.16   547   ◯                                                                     ◯                                                                     ◯                                                                     X   --                             315 600    Good   1.21   608   ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                           X                              316*                                                                              650    Partially                                                                            1.39   211   ◯                                                                     X   --  --  --                                        flow                                                               317*                                                                              700    Partially                                                                            1.65    8    X   --  --  --  --                                        flow                                                               __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

EXAMPLE 4

Crystallized glass comprising PbO as a main component which containsTiO₂, and a zinc oxide varistor using the same as a materialconstituting a high resistive side layer will now be explained.

First, each predetermined amount of PbO, ZnO, B₂ O₃, SiO₂, and TiO₂ wasweighed, and then crystallized glass for coating was prepared accordingto the same process as that used in Example 1 above. The crystallizedglass thus obtained was evaluated for the glass transition point (Tg),the coefficient of linear expansion (α), and the crystallinity. Theresults are shown in Table 13 below.

                                      TABLE 13                                    __________________________________________________________________________    Name of                                                                            Composition (percent by weight)                                                               Tg  α                                                                             Crystal-                                       glass                                                                              PbO                                                                              ZnO                                                                              B.sub.2 O.sub.3                                                                   SiO.sub.2                                                                        TiO.sub.2                                                                        (°C.)                                                                      (10.sup.-7 /°C.)                                                             linity                                         __________________________________________________________________________    G401*                                                                              40 25  5  10 20 360 58    ◯                                  G402 50 25  5  10 10 363 68    ◯                                  G403 75 10  5  5  5  344 87    ◯                                  G404*                                                                              85 10  5  0  0  315 96    X                                              G405*                                                                              55 40  5  0  0  350 60    ◯                                  G406 55 30 10  0  5  361 66    ◯                                  G407 70  5 15  5  5  375 82    ◯                                  G408*                                                                              70  0 20  5  5  396 85    X                                              G409 67.5                                                                             20 10  0  2.5                                                                              382 83    ◯                                  G410 67.4                                                                             20 10  0.1                                                                              2.5                                                                              385 84    ◯                                  G411 62.5                                                                             20 10  5  2.5                                                                              392 78    ◯                                  G412 57.5                                                                             20 10  10 2.5                                                                              401 75    ◯                                  G413*                                                                              47.5                                                                             20 10  20 2.5                                                                              405 70    ◯                                  G414*                                                                              59.9                                                                             20 10  10 0.1                                                                              392 71    ◯                                  G415 59.5                                                                             20 10  10 0.5                                                                              400 73    ◯                                  G416 55 20 10  10 5  404 69    ◯                                  G417 50 20 10  10 10 408 68    ◯                                  G418*                                                                              45 20 10  10 15 420 65    ◯                                  __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

As shown in Table 13, the addition of a large amount of PbO raises thecoefficient of linear expansion (α), while the addition of a largeamount of ZnO lowers the glass transition point (Tg), which facilitatescrystallization of the glass composition. Conversely, the addition of alarge amount of B₂ O₃ raises the glass transition point, and theaddition of more than 15.0 percent by weight of B₂ O₃ causes difficultyin crystallization of the glass composition. Further, with an increasein the amount of SiO₂ added, the glass transition point tends toincrease, while the coefficient of linear expansion tends to decrease.With an increase in the amount of TiO₂ added, the crystallization ofglass proceeded. The glass composition comprising a small amount of PbOand B₂ O₃ tended to become porous.

Next, the aforesaid frit glass was made into paste, after which theresulting glass paste was applied to the sides of the sintered body ofExample 1, followed by baking treatment to prepare a sample of a zincoxide varistor in the same process as that used in Example 1 above.Thereafter, the resulting samples were evaluated for theircharacteristics. The results are shown in Table 14 below.

                                      TABLE 14                                    __________________________________________________________________________                         Life under                                                                          Discharge withstand current                        Name of              voltage                                                                             rating properties                                  glass  Appearance                                                                           V.sub.1mA /V.sub.10μA                                                             (Time)                                                                              40 kA                                                                             50 kA                                                                             60 kA                                                                             70 kA                                                                             80 kA                              __________________________________________________________________________    G401*  Peel off                                                                             1.16   480   X   --  --  --  --                                 G402   Good   1.21   420   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G403   Good   1.32   331   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G404*  Crack  1.55    15   X   --  --  --  --                                 G405*  Partially                                                                            1.31   181   Δ                                                                           X   --  --  --                                        Peel off                                                               G406   Good   1.24   295   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     X                                  G407   Good   1.20   316   ◯                                                                     ◯                                                                     X   --  --                                 G408*  Partially                                                                            1.35   202   X   --  --  --  --                                        crack                                                                  G409   Good   1.25   367   ◯                                                                     Δ                                                                           X   --  --                                 G410   Good   1.26   351   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G411   Good   1.25   410   ◯                                                                     ◯                                                                     ◯                                                                     X   --                                 G412   Good   1.20   530   ◯                                                                     ◯                                                                     X   --  --                                 G413*  Porous 1.19   366   ◯                                                                     X   --  --  --                                 G414*  Good   1.34   197   ◯                                                                     X   --  --  --                                 G415   Good   1.29   348   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G416   Good   1.17   435   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     X                                  G417   Good   1.15   650   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G418*  Porous 1.20   241   Δ                                                                           X   --  --  --                                 Conventional                                                                         Good   1.26   153   ◯                                                                     Δ                                                                           X   --  --                                 example                                                                       __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

The data shown in Tables 13 and 14 indicated that when the coefficientof linear expansion of glass for coating was smaller than 65×10⁻⁷ /° C.(G401, G405 glass), the glass tended to peel off, and when exceeding90×10⁻⁷ /° C. (G404 glass), the glass tended to crack. It is supposedthat the samples of glass which cracked or peeled off have poordischarge withstand current rating properties due to the inferiorinsulating properties of the high resistive side layer. However, even ifthe coefficient of linear expansion of glass for coating is within therange of 65×10⁻⁷ to 90×10⁻⁷ /° C., glass with poor crystallinity (G408glass) tends to crack and also has poor discharge withstand currentrating properties. This may be attributed to the fact that the coatingfilm of crystallized glass has higher strength than that of non-crystalglass.

The amount of TiO₂ added will now be considered. First, any compositionwith 0.5 percent by weight or more of TiO₂ added has the improvednon-linearity with respect to voltage, accompanied by the improved lifecharacteristics under voltage. This may be attributed to the fact thatthe addition of 0.5 percent by weight or more of TiO₂ raises theinsulation resistance of the coating film. On the other hand, theaddition of more than 10.0 percent by weight of TiO₂ lowers thedischarge withstand current rating properties. This may be attributed tothe fact that glass tends to become porous due to its poor fluidityduring the baking process. Consequently, a PbO-ZnO-B₂ O₃ -SiO₂ -TiO₂type crystallized glass composition for the high resistive side layer ofa zinc oxide varistor is required to comprise TiO₂ at least in an amountof 0.5 to 10.0 percent by weight.

The above results confirmed that the most preferable crystallized glasscomposition for coating comprised 50.0 to 75.0 percent by weight of PbO,10.0 to 30.0 percent by weight of ZnO, 5.0 to 10.0 percent by weight ofB₂ O₃, 0 to 15.0 percent by weight of SiO₂, and 0.5 to 10.0 percent byweight of TiO₂. A crystallized glass composition for the high resistiveside layer of a zinc oxide varistor is also required to havecoefficients of linear expansion in the range of 65×10⁻⁷ to 90×10⁻⁷ /°C.

Next, by the use of G406 glass shown as a sample of the presentinvention in Table 13, the amount of glass paste to be applied wasexamined. The results are shown in Table 15 below. Glass paste wasapplied in a ratio of 1.0 to 300.0 mg/cm², which was controlled by theviscosity and the number of application of the paste. As shown in Table15, when glass paste is applied in a ratio of less than 10.0 mg/cm², theresulting coating film has low strength, while with a ratio of more than150.0 mg/cm², glass tends to flow or have pinholes. Both cases result inpoor discharge withstand current rating properties. These resultsindicated that glass paste was applied most preferably in a ratio of10.0 to 150.0 mg/cm².

                                      TABLE 15                                    __________________________________________________________________________        Amount of           Life under                                                                          Discharge withstand current                     Sample                                                                            application         voltage                                                                             rating properties                               No. (mg/cm.sup.2)                                                                       Appearance                                                                           V.sub.1mA /V.sub.10μA                                                             (Time)                                                                              40 kA                                                                             50 kA                                                                             60 kA                                                                             70 kA                                                                             80 kA                           __________________________________________________________________________    401*                                                                               1    Good   1.11   314   X   --  --  --  --                              402*                                                                               5    Good   1.14   380   Δ                                                                           X   --  --  --                              403  10   Good   1.16   560   ◯                                                                     ◯                                                                     Δ                                                                           X   --                              404  50   Good   1.17   435   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     X                               405 150   Good   1.25   413   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     X                               406*                                                                              200   Partially                                                                            1.29   242   ◯                                                                     X   --  --  --                                        flow                                                                407*                                                                              300   Flow   1.36   191   Δ                                                                           X   --  --  --                              __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

Next, by the use of G406 glass shown as a sample of the presentinvention in Table 13, the conditions under which glass paste wassubjected to baking treatment were examined. The results are shown inTable 16 below. The viscosity and the number of application of glasspaste were controlled so that the glass paste may be applied in a ratioof 50.0 mg/cm². Glass paste was subjected to baking treatment attemperatures in the range of 350° to 700° C. for 1 hour in air. As aresult, when baking treatment was conducted at a temperature of lessthan 450° C., glass paste was not sufficiently melted, resulting in poordischarge withstand current rating properties. On the other hand, whenbaking treatment was conducted at a temperature of more than 600° C.,the voltage ratio markedly lowered, resulting in poor lifecharacteristics under voltage. These results indicated that glass pastewas subjected to baking treatment most preferably at temperatures in therange of 450° to 600° C.

                                      TABLE 16                                    __________________________________________________________________________        Temperature          Life under                                                                          Discharge withstand current                    Sample                                                                            of baking            voltage                                                                             rating properties                              No. (°C.)                                                                         Appearance                                                                           V.sub.1mA /V.sub.10μA                                                             (Time)                                                                              40 kA                                                                             50 kA                                                                             60 kA                                                                             70 kA                                                                             80 kA                          __________________________________________________________________________    411*                                                                              350    Not    1.10    45   X   --  --  --  --                                        sintered                                                           412*                                                                              400    Porous 1.13    40   Δ                                                                           X   --  --  --                             413 450    Good   1.15   241   ◯                                                                     ◯                                                                     X   --  --                             414 500    Good   1.16   492   ◯                                                                     ◯                                                                     ◯                                                                     X   --                             415 600    Good   1.23   650   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     --                             416*                                                                              650    Partially                                                                            1.34   206   ◯                                                                     X   --  --  --                                        flow                                                               417*                                                                              700    Partially                                                                            1.58    13   Δ                                                                           X   --  --  --                                        flow                                                               __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

EXAMPLE 5

Crystallized glass comprising PbO as a main component which containsNiO, and a zinc oxide varistor using the same as a material constitutinga high resistive side layer will now be explained.

First, each predetermined amount of PbO, ZnO, B₂ O₃, SiO₂, and NiO wasweighed, and then crystallized glass for coating was prepared accordingto the same process as that used in Example 1 above. The crystallizedglass thus obtained was evaluated for the glass transition point (Tg),the coefficient of linear expansion (α), and the crystallinity. Theresults are shown in Table 17 below.

                                      TABLE 17                                    __________________________________________________________________________    Name of                                                                            Composition (percent by weight)                                                               Tg  α                                                                             Crystal-                                       glass                                                                              PbO                                                                              ZnO                                                                              B.sub.2 O.sub.3                                                                   SiO.sub.2                                                                        NiO                                                                              (°C.)                                                                      (10.sup.-7 /°C.)                                                             linity                                         __________________________________________________________________________    G501*                                                                              50 25  5  10 10 354 59    ◯                                  G502 55 25  5  10 5  360 69    ◯                                  G503 75 10  5  5  5  346 88    ◯                                  G504 85 10  5  0  0  315 96    X                                              G505*                                                                              55 40  5  0  0  350 60    ◯                                  G506 55 30 10  0  5  359 68    ◯                                  G507 70  5 15  5  5  370 84    ◯                                  G508*                                                                              70  0 20  5  5  394 88    X                                              G509 67.5                                                                             20 10  0  2.5                                                                              380 85    ◯                                  G510 67.4                                                                             20 10  0.1                                                                              2.5                                                                              381 85    ◯                                  G511 62.5                                                                             20 10  5  2.5                                                                              393 78    ◯                                  G512 57.5                                                                             20 10  10 2.5                                                                              404 76    ◯                                  G513*                                                                              47.5                                                                             20 10  20 2.5                                                                              409 71    ◯                                  G514 59.9                                                                             20 10  10 0.1                                                                              393 72    ◯                                  G515 59.5                                                                             20 10  10 0.5                                                                              395 72    ◯                                  G516 57 20 10  10 2.5                                                                              405 70    ◯                                  G517 55 20 10  10 5  406 69    ◯                                  G518*                                                                              50 20 10  10 10 415 63    ◯                                  __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

As shown in Table 17, the addition of a large amount of PbO raises thecoefficient of linear expansion (α), while the addition of a largeamount of ZnO lowers the glass transition point (Tg), which facilitatescrystallization of the glass composition. Conversely, the addition of alarge amount of B₂ O₃ raises the glass transition point, and theaddition of more than 15.0 percent by weight of B₂ O₃ causes difficultyin crystallization of the glass composition. Further, with an increasein the amount of SiO₂ added, the glass transition point tends toincrease, while the coefficient of linear expansion tends to decrease.With an increase in the amount of NiO added, the crystallization ofglass proceeded. The glass composition comprising a small amount of PbOand B₂ O₃ tended to become porous.

Next, the aforesaid frit glass was made into paste, after which theresulting glass paste was applied to the sides of the sintered body ofExample 1, followed by baking treatment to prepare a sample of a zincoxide varistor in the same process as that used in Example 1 above.Thereafter, the resulting samples were evaluated for theircharacteristics. The results are shown in Table 18 below.

                                      TABLE 18                                    __________________________________________________________________________                         Life under                                                                          Discharge withstand current                        Name of              voltage                                                                             rating properties                                  glass  Appearance                                                                           V.sub.1mA /V.sub.10μA                                                             (Time)                                                                              40 kA                                                                             50 kA                                                                             60 kA                                                                             70 kA                                                                             80 kA                              __________________________________________________________________________    G501*  Peel off                                                                             1.15   490   X   --  --  --  --                                 G502   Good   1.20   440   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G503   Good   1.33   331   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G504*  Crack  1.55    15   X   --  --  --  --                                 G505*  Partially                                                                            1.31   181   Δ                                                                           X   --  --  --                                        peel off                                                               G506   Good   1.25   288   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     X                                  G507   Good   1.22   340   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G508*  Partially                                                                            1.34   207   X   --  --  --  --                                        crack                                                                  G509   Good   1.25   335   ◯                                                                     Δ                                                                           X   --  --                                 G510   Good   1.28   384   ◯                                                                     ◯                                                                     ◯                                                                     X   --                                 G511   Good   1.27   411   ◯                                                                     ◯                                                                     ◯                                                                     X   --                                 G512   Good   1.24   492   ◯                                                                     ◯                                                                     X   --  --                                 G513*  Porous 1.18   375   Δ                                                                           X   --  --  --                                 G514*  Good   1.33   209   ◯                                                                     X   --  --  --                                 G515   Good   1.29   394   ◯                                                                     ◯                                                                     Δ                                                                           X   --                                 G516   Good   1.18   482   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     Δ                            G517   Good   1.16   591   ◯                                                                     ◯                                                                     ◯                                                                     Δ                                                                           X                                  G518*  Porous 1.23   205   Δ                                                                           X   --  --  --                                 Conventional                                                                         Good   1.26   153   ◯                                                                     Δ                                                                           X   --  --                                 example                                                                       __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

The data shown in Tables 17 and 18 indicated that when the coefficientOf linear expansion of glass

for coating was smaller than 65×10⁻⁷ /° C. (G501, G505 glass), the glasstended to peel off, and when exceeding 90×10⁻⁷ /° C. (G504 glass), theglass tended to crack. It is supposed that the samples of glass whichcracked or peeled off have poor discharge withstand current ratingproperties due to the inferior insulating properties of the highresistive side layer. However, even if the coefficient of linearexpansion of glass for coating is within the range of 65×10⁻⁷ to 90×10⁻⁷/° C., glass with poor crystallinity (G508 glass) tends to crack andalso has poor discharge withstand current rating properties. This may beattributed to the fact that the coating film of crystallized glass hashigher strength than that of non-crystal glass.

The amount of NiO added will now be considered. First, any compositionwith 0.5 percent by weight or more of NiO added has the improvednon-linearity with respect to voltage, accompanied by the improved lifecharacteristics under voltage. This may be attributed to the fact thatthe addition of 0.5 percent by weight or more of NiO raises theinsulation resistance of the coating film. On the other hand, theaddition of more than 5.0 percent by weight of NiO lowers the dischargewithstand current rating properties. This may be attributed to the factthat glass tends to become porous due to its poor fluidity during bakingprocess. Consequently, a PbO-ZnO-B₂ O₃ -SiO₂ -NiO type crystallizedglass composition for the high resistive side layer of a zinc oxidevaristor is required to comprise NiO at least in an amount of 0.5 to 5.0percent by weight.

The above results confirmed that the most preferable crystallized glasscomposition for coating comprised 55.0 to 75.0 percent by weight of PbO,10.0 to 30.0 percent by weight of ZnO, 5.0 to 10.0 percent by weight ofB₂ O₃, 0 to 15.0 percent by weight of SiO₂, and 0.5 to 5.0 percent byweight of NiO. A crystallized glass composition for the high resistiveside layer of a zinc oxide varistor is also required to havecoefficients of linear expansion in the range of 65×10⁻⁷ to 90×10⁻⁷ /°C.

Next, by the use of G516 glass shown as a sample of the presentinvention in Table 17, the amount of glass paste to be applied wasexamined. The results are shown in Table 19 below. Glass paste wasapplied in a ratio of 1.0 to 300.0 mg/cm², which was controlled by theviscosity and the number of application of the paste. In this process,when glass paste is applied in a ratio of less than 10.0 mg/cm², theresulting coating film has low strength, while with a ratio of more than150.0 mg/cm², glass tends to flow or have pinholes. Both cases result inpoor discharge withstand current rating properties. These resultsindicated that glass paste was applied most preferably in a ratio of10.0 to 15.0 mg/cm².

                                      TABLE 19                                    __________________________________________________________________________        Amount of           Life under                                                                          Discharge withstand current                     Sample                                                                            application         voltage                                                                             rating properties                               No. (mg/cm.sup.2)                                                                       Appearance                                                                           V.sub.1mA /V.sub.10μA                                                             (Time)                                                                              40 kA                                                                             50 kA                                                                             60 kA                                                                             70 kA                                                                             80 kA                           __________________________________________________________________________    501*                                                                               1    Good   1.12   300   X   --  --  --  --                              502  5    Good   1.14   391   ◯                                                                     X   --  --  --                              503  10   Good   1.17   567   ◯                                                                     ◯                                                                     ◯                                                                     X   --                              504  50   Good   1.18   482   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     Δ                         505 150   Good   1.26   318   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     X                               506*                                                                              200   Partially                                                                            1.29   209   ◯                                                                     X   --  --  --                                        flow                                                                507*                                                                              300   Flow   1.38   154   Δ                                                                           X   --  --  --                              __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

Next, by the use of G516 glass shown as a sample of the presentinvention in Table 17, the conditions under which glass paste wassubjected to baking treatment were examined. The results are shown inTable 20 below. The viscosity and the number of application of glasspaste were controlled so that the glass paste may be applied in a ratioof 50.0 mg/cm². Glass paste was subjected to baking treatment attemperatures in the range of 350° to 700° C. for 1 hour in air. As aresult, when baking treatment was conducted at a temperature of lessthan 450° C., glass paste was not sufficiently melted, resulting in poordischarge withstand current rating properties. On the other hand, whenbaking treatment was conducted at a temperature of more than 60° C., thevoltage ratio markedly lowered, resulting in poor life characteristicsunder voltage. These results indicated that glass paste was subjected tobaking treatment most preferably at temperatures in the range of 450° to600° C.

                                      TABLE 20                                    __________________________________________________________________________        Temperature          Life under                                                                          Discharge withstand current                    Sample                                                                            of baking            voltage                                                                             rating properties                              No. (°C.)                                                                         Appearance                                                                           V.sub.1mA /V.sub.10μA                                                             (Time)                                                                              40 kA                                                                             50 kA                                                                             60 kA                                                                             70 kA                                                                             80 kA                          __________________________________________________________________________    511*                                                                              350    Not    1.11    40   X   --  --  --  --                                        sintered                                                           512*                                                                              400    Porous 1.14    32   Δ                                                                           X   --  --  --                             513 450    Good   1.14   251   ◯                                                                     ◯                                                                     X   --  --                             514 500    Good   1.17   483   ◯                                                                     ◯                                                                     ◯                                                                     X   --                             515 600    Good   1.25   644   ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     X                              516*                                                                              650    Partially                                                                            1.33   217   ◯                                                                     X   --  --  --                                        flow                                                               517*                                                                              700    Partially                                                                            1.54    12   Δ                                                                           X   --  --  --                                        flow                                                               __________________________________________________________________________     A mark "*" denotes a control sample which is not within the scope of the      present invention.                                                       

As typical examples of crystallized glass comprising PbO as a maincomponent, described are four-components type such as PbO-ZnO-B₂ O₃-SiO₂ in Example 1 above, four-components type such as PbO-ZnO-B₂ O₃-MoO₃, and five-components type such as PbO-ZnO-B₂ O₃ -SiO₂ -MoO₃ inExample 2, five-components type such as PbO-ZnO-B₂ O₃ -SiO₂ -WO₃ inExample 3, four-components type such as PbO-ZnO-B₂ O₃ -TiO₂, andfive-components type such as PbO-ZnO-B₂ O₃ -SiO₂ -TiO₂ in Example 4, andfour-components type such as PbO-ZnO-B₂ O₃ -NiO and five-components typesuch as PbO-ZnO-B₂ O₃ -SiO₂ -NiO in Example 5. The effect of the presentinvention may not vary according to the addition of an additive whichfurther facilitates crystallization of glass such as Al₂ O₃ or SnO₂.

As a substance for lowering the glass transition point, ZnO was used inthe above examples, and it is needless to say that other substances suchas V₂ O₅ which are capable of lowering the glass transition point mayalso be used as a substitute thereof. Further, as a typical example ofan oxide ceramic, crystallized glass for coating comprising PbO as amain component of the present invention is used for a zinc oxidevaristor in the examples of the present invention. This crystallizedglass may be applied quite similarly to any oxide ceramics employed fora strontium titanate type varistor, a barium titanate type capacitor, aPTC thermistor, or a metallic oxide type NTC thermistor.

Industrial Applicability

As indicated above, the present invention can provide a zinc oxidevaristor excellent in the non-linearity with respect to voltage, thedischarge withstand current rating properties, and the lifecharacteristics under voltage by using various PbO type crystallizedglass with high crystallinity and strong coating film as a materialconstituting the high resistive side layer formed on a sintered bodycomprising zinc oxide as a main component. A zinc oxide varistor of thepresent invention has very high availability as a characteristic elementof an arrestor for protecting a transmission and distribution line andperipheral devices thereof requiring high reliability from surge voltagecreated by lightning.

Crystallized glass for coating comprising PbO as a main component of thepresent invention may be used as a covering material for not only a zincoxide varistor but also various oxide ceramics employed for a strontiumtitanate type varistor, a barium titanate type capacitor, a positivethermistor, etc., and a metallic oxide type negative thermistor and aresistor to enhance the strength and stabilize or improve the variouselectric characteristics thereof. Moreover, apparent from aboveexamples, conventional glass for coating tends to have a porousstructure because it is composite glass containing feldspar, whereas thePbO type crystallized glass of the present invention is also capable ofimproving the chemical resistance and the moisture resistance due to thehigh crystallinity and the tendency to have a uniform and closestructure, thereby promising many very useful applications.

We claim:
 1. A zinc oxide varistor comprising a sintered body containingzinc oxide as a main component and having varistor characteristics, anda high resistive side layer formed on the sides of the sintered body,the side layer consisting of crystallized glass consisting of 50.0 to75.0 percent by weight of PbO, 10.0 to 10.0 percent by weight of ZnO,5.0 to 10.0 percent by weight of B₂ O₃, and 6.0 to 15.0 percent byweight of SiO₂.
 2. A zinc oxide varistor comprising a sintered bodycontaining zinc oxide as a main component and having varistorcharacteristics, and a high resistive side layer formed on the sides ofthe sintered body, the side layer consisting of crystallized glasscomprising PbO as a main component which contains at least 0.1 to 10.0percent by weight of molybdenum oxide calculated in terms of MoO₃.
 3. Azinc oxide varistor according to claim 2, wherein said high resistiveside layer consists of PbO-ZnO-B₂ O₃ -MoO₃ type crystallized glass.
 4. Azinc oxide varistor according to claim 2, wherein said high resistiveside layer consists of PbO-ZnO-B₂ O₃ -SiO₂ -MoO₃ type crystallizedglass.
 5. A zinc oxide varistor according to claim 2, wherein said highresistive side layer consists of crystallized glass comprising 50.0 to75.0 percent by weight of PbO, 10.0 to 30.0 percent by weight of ZnO,5.0 to 15.0 percent by weight of B₂ O₃, 0 to 15.0 percent by weight ofSiO₂, and 0.1 to 10.0 percent by weight of MoO₃.
 6. A zinc oxidevaristor comprising a sintered body containing zinc oxide as a maincomponent and having varistor characteristics, and a high resistive sidelayer formed on the sides of the sintered body, the side layerconsisting of crystallized glass comprising PbO as a main componentwhich contains at least 0.5 to 10.0 percent by weight of WO₃.
 7. A zincoxide varistor according to claim 6, wherein said high resistive sidelayer consists of PbO-ZnO-B₂ O₃ -SiO₂ -WO₃ type crystallized glass.
 8. Azinc oxide varistor according to claim 6, wherein said high resistiveside layer consists of crystallized glass comprising 50.0 to 75.0percent by weight of PbO, 10.0 to 30.0 percent by weight of ZnO, 5.0 to15.0 percent by weight of B₂ O₃, 0.5 to 15.0 percent by weight of SiO₂,and 0.5 to 10.0 percent by weight of WO₃.
 9. A zinc oxide varistorcomprising a sintered body containing zinc oxide as a main component andhaving varistor characteristics, and a high resistive side layer formedon the sides of the sintered body, the side layer consisting ofcrystallized glass comprising PbO as a main component which contains atleast 0.5 to 10.0 percent by weight of titanium oxide calculated interms of TiO₂.
 10. A zinc oxide varistor according to claim 9, whereinsaid high resistive side layer consists of PbO-ZnO-B₂ O₃ -TiO₂ typecrystallized glass.
 11. A zinc oxide varistor according to claim 9,wherein said high resistive side layer consists of PbO-ZnO-B₂ O₃ -SiO₂-TiO₂ type crystallized glass.
 12. A zinc oxide varistor according toclaim 9, wherein said high resistive side layer consists of crystallizedglass comprising 50.0 to 75.0 percent by weight of PbO, 10.0 to 30.0percent by weight of ZnO, 5.0 to 15.0 percent by weight of B203, 0 to15.0 percent by weight of SiO₂, and 0.5 to 10.0 percent by weight ofTiO₂.
 13. A zinc oxide varistor comprising a sintered body containingzinc oxide as a main component and having varistor characteristics, anda high resistive side layer formed on the sides of the sintered body,the side layer consisting of crystallized glass comprising PbO as a maincomponent which contains at least 0.5 to 5.0 percent by weight of nickeloxide calculated in terms of NiO.
 14. A zinc oxide varistor according toclaim 13, wherein said high resistive side layer consists of PbO-ZnO-B₂O₃ -NiO type crystallized glass.
 15. A zinc oxide varistor according toclaim 13, wherein said high resistive side layer consists of PbO-ZnO-B₂O₃ -SiO₂ -NiO type crystallized glass.
 16. A zinc oxide varistoraccording to claim 13, wherein said high resistive side layer consistsof crystallized glass comprising 55.0 to 75.0 percent by weight of PbO,10.0 to 30.0 percent by weight of ZnO, 5.0 to 15.0 percent by weight ofB₂ O₃, 0 to 15.0 percent by weight of SiO₂, and 0.5 to 5.0 percent byweight of NiO.